Chainsaws Don't Kill People; People Kill People
Suddenly, the wall behind Mary and Andy slides open to reveal Leatherface, who revs up his chainsaw and gets ready for action. Yeah, Leatherface! Go for the girl first! She's slower and weaker! Wheelchair trips Andy and Mary up with his plot-contrivance cane, and they fall over. Mary kicks a wooden door closed to keep Leatherface out, which it does for all of two seconds. Andy wastes precious seconds trying to pick his tire iron up from off of the floor, which even Mary knows is incredibly stupid, so she abandons him and gets the hell out of the house. This leaves Andy alone to fend off the chainsaw-wielding maniac with his precious tire iron, which he manages to do well enough to get out of the house. Leatherface follows, showing a remarkable amount of finesse when it comes to turning around in tight space with a chainsaw. I'm sure he's had some practice, though.
Outside, Andy stupidly runs right into a maze of sheets drying in the sun. Leatherface is right behind him. Andy gets lost and confused in the sheets, which gets Leatherface close enough to cut off his leg. Andy writhes on the ground in agony until Leatherface picks up him in a fireman's carry and walks towards the house. I don't know why Andy's so upset about losing a limb; in this movie, people without legs can do everything that people with legs can do, even if it's physically impossible!
Mary whines and whimpers as she runs through the Forest of Doom, but I don't know what she's complaining about, as all her limbs are still attached to her body.
Leatherface carries Andy into the basement. Andy screams and scratches at the walls, getting his fingernail ripped off. Just in case you didn't remember that from earlier in the film, it's shown in slow-motion in a disgusting close-up.